Improve timeslot conflict resolution & deletion of single schedule with two timeslots

858f074a already implements a visual way of switching between conflict resolution modes. But it has to be improved for the ours-.* modes.

Also there is a problem when we create a schedule only for a single emission and then have two resulting timeslots in the new schedule, because a theirs-both mode was chosen. In that case, the timeslot deletion modal informs us that there this is the only timeslot in the schedule and then deletes the whole schedule (including the other timeslot).